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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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    ST-120 Questions

    Bill
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    Bill


    Posts : 1
    Join date : 2010-03-11
    • Post n°1

    ST-120 Questions Empty ST-120 Questions

    Post by Bill Thu Mar 11, 2010 6:13 pm

    I have read that when you increase the power supply filter capacitor value or run a solid state rectifier that you need a standby mode to allow the tubes to warm up before applying high voltage to prevent the cathodes of the power output tubes from cathode stripping.

    Also that tube rectification has better bass but concerned that the rectifier tube is running at very close to maximum output and concerned about tube life.
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    Bob Latino
    Bob Latino
    Bob Latino
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    Posts : 3262
    Join date : 2008-11-26
    Location : Massachusetts
    • Post n°2

    ST-120 Questions Empty Re: ST-120 Questions

    Post by Bob Latino Thu Mar 11, 2010 7:08 pm

    Bill wrote:I have read that when you increase the power supply filter capacitor value or run a solid state rectifier that you need a standby mode to allow the tubes to warm up before applying high voltage to prevent the cathodes of the power output tubes from cathode stripping.

    Also that tube rectification has better bass but concerned that the rectifier tube is running at very close to maximum output and concerned about tube life.

    Hi Bill,

    A GZ34 tube rectifier will work fine in the ST-120. If you play the amp loud a lot or have inefficient speakers then I recommend the Weber Copper Cap solid state rectifier model WZ68 for the ST-120. The Weber has a short delay built in before it applies high voltage similar to a tube rectifier. The delay is not as long as a tube rectifier but it works well. At higher current demands the Weber can flow up to 450 milliamps of current. This much more than any tube rectifier can flow. I have used a Weber WZ68 in my own ST-120 for about 2 years now and have probably near 2000 hours on this amp and both the Weber and the four Genalex Gold Lion KT88 output tubes are still running fine.

    I am in disageement with your second statement about better bass with the tube rectifier. Listening to the ST-120 with both rectifiers I find that the Weber has better bass AT HIGHER VOLUME LEVELS. At normal or lower volume levels I don't hear any diffrence between the bass response of either rectifier - tube or solid state. At very high volume levels any tube rectifier (even a Mullard) will "sag" the B+ high voltage more than a solid state rectifier and will eventually "run out of gas" before a solid state rectifier will.

    Re: Your mention of "cathode stripping" on output tubes. The Dynaco ST-35 power amp and the SCA-35 integrated amp both had solid state rectifiers with NO DELAY before applying high voltage. The fact that literally thousands of these amps are still in service and still running fine (some WITH THEIR ORIGINAL OUTPUT TUBES) after 40+ years leads me to believe that idea of "cathode stripping" of output tubes on amps with solid state rectification may be somewhat overstated.

    Bob
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